Rotary hammers are known which have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit. Such hammers are generally provided with an impact mechanism which converts the rotational drive from an electric motor to a reciprocating drive causing a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of a closed air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer, optionally via a beatpiece.
Some hammers can be employed in combination impact and drilling mode or in a drilling only mode in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein will be caused to rotate. In the combination impact and drilling mode the bit will be caused to rotate at the same time as the bit receives repeated impacts. Such hammers generally have a hammer only mode in which the spindle is locked against rotation.
In some known designs of rotary hammer, for example in DE27 28 961, an axially moveable spindle drive gear may be mounted non-rotatably around the spindle. The axial position of the spindle drive gear is selected via a mode change mechanism actuated by a mode change knob. In a first axial position the gear engages an intermediate drive shaft in order to transfer rotary drive from the intermediate drive shaft to the hollow spindle. The first axial position is a hammer drilling or drilling only mode of the hammer. In a second axial position the gear is disengaged from the intermediate drive shaft and so no longer transfers said rotary drive. In the second position the gear engages a set of spindle lock teeth fixed inside the housing of the hammer, so as to rotationally fix the gear and thereby the spindle in the housing. The second position is a hammer only mode of the hammer.
One problem with such mode change mechanisms is gear synchronisation. In order to overcome this problem the gear may be biased into its first position, so that when the sleeve or gear is moved into the first position towards the intermediate drive shaft, if the sets of teeth on the gear and on the drive shaft are mis-aligned, as soon as the hammer is turned on and the drive shaft begins to rotate, the sets of teeth are brought into engagement by the biasing means as soon as the sets of teeth become aligned. Thus, it is relatively easy to overcome this synchronisation problem on entry into a rotary mode of the hammer.
When the sleeve or gear is moved into its second position, if the sets of teeth on the gear and on the spindle lock teeth are not aligned, they will not engage. This problem can be reduced to some extent by chamfering the sets of teeth. However, some manual adjustment of the rotational position of the spindle by a user is often required to bring the sets of teeth into engagement so that the spindle is locked.